The invention relates to a device for measuring hydrogen concentration in an aluminum melt by means of an immersion probe that is permeable to hydrogen and is made of melt-resistant, sintered or baked ceramic material connected in a vacuum tight manner, via a fitting capilliary tube of melt-resistant material that is impermeable to hydrogen, to a pressure gauge situated in a measuring head. In aluminum and aluminum alloys the solubility of hydrogen differs greatly depending on the state and temperature of the metal. Aluminum melts readily take up hydrogen from furnace gases, humid air and furnace linings. With the addition of alloying elements such as Mg for example hydrides and hydrates are also taken up by the melt in addition to the dissolved hydrogen; these compounds increase the hydrogen content of the melt further. The solubility of hydrogen decreases drastically during solidification from about 0,8 to 0,04 cm.sup.3 per 100 g of metal. The excess hydrogen precipitates out during solidification in the form of extremly fine pores which frequently diminish the mechanical properties of the finished product.
The demand for superior quality makes continuous supervision of the hydrogen content of the melt indispensible. Such supervision is usefully employed to check the metal quality just before the casting machine, to check the efficiency of melt treatment (degassing, removal of impurities with active gases) and to test insulating material for the release of hydrogen.
At present the methods for H.sub.2 -determination mentioned below are employed with various degrees of success. These methods make use of Sievert's Law which relates, under equilibrium conditions, the H.sub.2 concentration of the melt to the H.sub.2 -partial pressure in a gas bubble and the melt temperature according to the following equation: EQU c=k.sub.1 .sqroot.p.multidot.e.sup.-k.sbsp.2.sup./T
where:
c=H.sub.2 concentration in the melt (cm.sup.3 /100 g melt) PA0 p=H.sub.2 equilibrium pressure (mbar) PA0 T=absolute temperature of the melt PA0 k.sub.1, k.sub.2 =constants.
In one known process a circulating stream of nitrogen is introduced into the melt. When equilibrium saturation is reached, the hydrogen concentration is measured via a thermal conductivity cell. The high cost of the probes, which are susceptible to malfunctioning, and the need for trained personnel to operate them, limit the extent to which the instrument can be used in rough operating conditions.
Also known is to observe a melt sample in a transparent vacuum chamber under conditions of falling pressure. When the pressure becomes less than the equilibrium pressure of hydrogen in the melt, then bubbles form below the oxide skin. The instrument is robust enough for production conditions. The measurement, however, is subjective, and the method can not be employed with low H.sub.2 concentrations (.ltoreq.0,10 cm.sup.3 /100 g).
In another known hot-extraction method, samples that have been carefully prepared by machining are degassed in an inert gas stream either in the solid state at 500.degree.-600.degree. C. or after melting in vacuum. The process is labor intensive and therefore is expensive and can only be performed under laboratory conditions. The method is not suitable for magnesium-containing alloys because of getter effects and decomposition of hydride and hydrate.
In a further known method an immersion probe made of graphite and permeable to hydrogen but not the melt is dipped into the melt; the said probe is connected to a pressure measuring cell via a capillary tube. Impurity gases are first evacuated by means of a vacuum pump. The atomic hydrogen dissolved in the melt is recombined to hydrogen molecules on the graphite wall as in a gas bubble, and diffuses in the hollow body. The hydrogen content of the melt is then calculated via Sievert's Law from the equilibrium pressure of hydrogen and the melt temperature.